Braking hydraulic circuit for controlling linear reduction of front/rear wheel breaking liquid pressure and solving difference in gradient of front/rear wheel breaking pressure in vehicle

ABSTRACT

A breaking hydraulic circuit for linear pressure reduction of a front/rear wheel breaking liquid pressure in a vehicle includes a liquid pressure return line formed between wheel cylinders and a master cylinder, and a two-system liquid pressure control circuit separately passing liquid pressures to be discharged from the wheel cylinders and then unifies the liquid pressures. The breaking hydraulic circuit has normal open type solenoid valves and normal close type solenoid valves. In the breaking hydraulic circuit, linear control of a front/rear wheel breaking liquid pressure is realized at a time of implementation of ABS (Antilock Brake System) or TCS (Traction Control System), and a difference in pressure intensification gradient according to a difference in generation time between the front and rear wheels is eliminated.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on, and claims priority from, KoreanApplication Serial Number 10-2006-0123455, filed on Dec. 7, 2006, thedisclosure of which is hereby incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The present invention relates to a breaking hydraulic circuit for avehicle, and in particular, to a breaking hydraulic circuit forcontrolling linear reduction of a front/rear wheel breaking liquidpressure and solving a difference in gradient of a front/rear wheelbreaking liquid pressure in a vehicle.

BACKGROUND OF THE PRESENT INVENTION

In general, a breaking system that breaks a running vehicle using ahydraulic pressure for increasing a driver's control force of a breakpedal adopts a simple method that holds rotating wheels to stop therotation. In this case, however, it is difficult to implement optimumbreaking performance according to running states of the vehicle and roadconditions.

Accordingly, there are used various technologies and methods thatcontrol a breaking liquid pressure at the time of breaking in order toovercome the limitation of the simple breaking system and to improvesafety of the vehicle.

For example, the following systems can be applied: ABS (Antilock BrakeSystem) that appropriately adjusts a braking pressure to be applied towheels according to a slip ratio calculated from a wheel speed, therebypreventing locking of the wheels; TCS (Traction Control System) thatadjusts a driving force of an engine in order to prevent an excessiveslip at the time of sudden start or rapid acceleration of the vehicle;and ESP (Electronic Stability Program) that minimizes a differencebetween a desired vehicle running direction and an actual vehiclerunning direction and keeps a vehicle running direction according to adriver's intension under any driving conditions.

Such a breaking pressure control is implemented by providing varioussolenoid valves on a hydraulic circuit formed between a wheel cylinderfor holding and restricting a disk wheel and a master cylinder forgenerating a hydraulic pressure, and controlling a path from the mastercylinder to the wheel cylinder and a return path from the wheel cylinderto master cylinder.

At this time, a controller (typically, an ECU (Engine Control Unit) isapplied) is used for liquid pressure control on the hydraulic circuit,in addition to control of electrical components for implementing abreaking operation. That is, the controller reads the wheel speed andthe like, then performs arithmetic, analysis, and judgment according toan internal control program, and subsequently performs the pressurecontrol on a hydraulic line by driving an oil pump or the like, inaddition to on/off control of the solenoid valves.

However, with the simple on/off control of the solenoid valves, at thetime of implementing the ABS or the TCS, there is a limitation in that abreaking pressure cannot be controlled to have a desired pressuregradient. For this reason, in order to implement precise liquid pressurecontrol, there is needed a technology that controls the solenoid valvesin a PWM (Pulse Width Modulation) method, and implements a breakingpressure to have a pressure gradient according to a breaking state of avehicle at the time of breaking.

Meanwhile, in the PWM control, known on/off type solenoid valves cannotbe applied, and thus it is necessary to separately develop solenoidvalves that can be controlled to perform a desired operation in the PWMmethod. For this reason, costs for development and mass production maybe required. Then, upon application, competitiveness may be weakened interms of the costs.

In addition, at the time of normal breaking, that is, when the ABS orthe TCS does not operate, there is a phenomenon that a difference inpressure intensification gradient occurs due to a difference ingeneration time of a breaking pressure between front and rear wheels. Inthis case, even though PWM control type solenoid valves are applied tothe breaking hydraulic circuit, the above phenomenon is not solved.Accordingly, there is a limitation in that unstability at the time ofbreaking cannot be thoroughly solved.

SUMMARY OF THE PRESENT INVENTION

An embodiment of the present invention helps to overcome the drawbacksinherent in the related art and provide a breaking hydraulic circuitthat, even though an on/off type solenoid valve is applied, can controla pressure gradient according to a breaking state of a vehicle withoutrequiring time and costs for development of a separate PWM type solenoidvalve, with implementation of a function of controlling a pressurereduction gradient according to a PWM method when the ABS operates.

Another embodiment of the present invention provides a breakinghydraulic circuit. The breaking hydraulic circuit can perform a breakingoperation during pressure reduction control of an ABS or a TCS byproviding an accumulator for temporarily storing a discharged liquidpressure while forming normal close type solenoid valves double, whichform a two-system liquid pressure flow for separately passing liquidpressures to be discharged from wheel cylinders and then unifying theliquid pressures, on a liquid pressure return line formed between wheelcylinders and a master cylinder. As a result, it is possible toeliminate a difference in pressure intensification gradient according toa difference in generation time of a breaking pressure between front andrear wheels and thus solving breaking unstability.

A breaking hydraulic circuit according to an embodiment of the presentinvention includes a liquid pressure supply line, supply path controlvalves, a liquid pressure return line, return path control valves, a TCSliquid pressure supply line, and a controller.

The liquid pressure supply line supplies a liquid pressure from a mastercylinder to wheel cylinders of wheels at the time of breaking accordingto an operation of a brake pedal by a driver and at the time of normalbreaking with no application of an ABS.

The supply path control valves are normal open type solenoid valves andare provided on the liquid pressure supply line so as to control a flowof a hydraulic pressure to be supplied from the master cylinder to thewheel cylinders of the wheels.

The liquid pressure return line is formed so as to control liquidpressures to be discharged from the wheel cylinders at the time ofpressure reduction control with the application of the ABS.

The return path control valves are normal close type solenoid valves andare provided on the liquid pressure return line so as to form atwo-system liquid pressure flow for separately passing the liquidpressures to be discharged from the wheel cylinders and then unifyingthe liquid pressure. Accordingly, it is possible to eliminate adifference in pressure intensification gradient according to adifference in generation time of a breaking pressure between the frontand rear wheels at the time of the pressure reduction control.

The TCS liquid pressure supply line shares the liquid pressure supplyline and supplies the liquid pressure of the master cylinder to thewheel cylinders through a high pressure switching valve, a motor, and asuction pump at the time of implementation of a TCS for slipping controlwith no operation of the break pedal.

The controller controls the valves provided on the hydraulic circuit,the motor, and the suction pump on the basis of wheel speed informationmeasured by a wheel speed sensor at the time of normal breaking, the ABSpressure reduction control, or the implementation of the TCS.

The liquid pressure supply line may include a head liquid pressuresupply line, a front left wheel liquid pressure supply line, and a rearright wheel liquid pressure supply line. The head liquid pressure supplyline extends from the master cylinder. The front left wheel liquidpressure supply line is connected to the head liquid pressure supplyline and a wheel cylinder of a front left wheel, and has aunidirectional check valve so as to receive the liquid pressure from themaster cylinder. The rear right wheel liquid pressure supply linebranches off from the head liquid pressure supply line, to which thefront left wheel liquid pressure supply line is connected, is connectedto a wheel cylinder of a rear right wheel, and has a unidirectionalcheck valve so as to receive the liquid pressure from the mastercylinder.

A front wheel supply path control valve may be provided on the frontleft wheel liquid pressure supply line so as to open/close the path, anda rear wheel supply path control valve may be provided on the rear rightwheel liquid pressure supply line so as to open/close the path.

The liquid pressure supply line may be provided with a pressure settingvalve that, when the liquid pressure exceeds a prescribed liquidpressure, returns an excessive liquid pressure to the master cylinder soas to keep the entire hydraulic line to a constant liquid pressure. Thepressure setting valve may be provided on the head liquid pressuresupply line before the front left wheel liquid pressure supply line andthe rear right wheel liquid pressure supply line branch off.

The liquid pressure return line may include wheel cylinder liquidpressure return lines, a unified liquid pressure return line, aconnection liquid pressure return line, and a discharge liquid pressurereturn line. The wheel cylinder liquid pressure return lines extend fromthe wheel cylinder of the front left wheel and the wheel cylinder of therear right wheel, and have unidirectional check valves, respectively.The unified liquid pressure return line is formed by unifying the wheelcylinder liquid pressure return lines. The connection liquid pressurereturn line is connected between the unified liquid pressure return lineand a suction pump, and has a unidirectional check valve, and to whichan accumulator for temporarily storing the liquid pressure is connected.The discharge liquid pressure return line is connected to the liquidpressure supply line connected to the master cylinder so as to returnthe liquid pressure pumped from the suction pump to the master cylinder.

The discharge liquid pressure return line may be connected to the liquidpressure supply lines that are respectively formed from master cylinderto the wheel cylinders of the front left wheel and rear right wheel.Further, the discharge liquid pressure return line may be locatedbetween the front wheel and rear wheel supply path control valvesprovided on the liquid pressure supply lines and the pressure settingvalve.

Front wheel and rear wheel return path control valves may be provided inthe wheel cylinder liquid pressure return lines, respectively, so as toopen/close the paths, and a unified return path control valve may beprovided on the connection liquid pressure return line connected to theunified liquid pressure return line so as to open/close the path.

The TCS liquid pressure supply line may branch off from the liquidpressure supply line extending from the master cylinder, may beconnected to the liquid pressure return line connected to a previousstage of the suction pump, and may be connected to the liquid pressuresupply line through the liquid pressure supply line extending from thesuction pump, thereby forming the hydraulic circuit.

As the breaking hydraulic circuit formed between the master cylinder andthe wheel cylinders of the front left wheel and rear right wheel, abreaking hydraulic circuit may be formed between the master cylinder andwheel cylinders of a front right wheel and a rear left wheel, and thesuction pumps respectively provided in the breaking hydraulic circuitsmay be driven by the single motor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of the presentinvention, reference should be made to the following detaileddescription with the accompanying drawings, in which:

FIG. 1 is a diagram showing the configuration of a breaking hydrauliccircuit for controlling linear pressure reduction of a front/rear wheelbreaking liquid pressure and solving a difference in gradient of thefront/rear wheel breaking liquid pressure according to an embodiment ofthe present invention;

FIG. 2 is a diagram of a flow of a liquid pressure at the time of normalbreaking with no application of an ABS in a case where the breakinghydraulic circuit according to the embodiment of the present inventionis used;

FIG. 3 is a diagram showing a flow of a liquid pressure at the time ofpressure reduction control with the application of the ABS in a casewhere the breaking hydraulic circuit according to the embodiment of thepresent invention is used;

FIG. 4 is a diagram showing a flow of a liquid pressure when a breakingoperation is performed again after the pressure reduction control withthe application of the ABS in a case where the breaking hydrauliccircuit according to the embodiment of the present invention is used;and

FIG. 5 is a diagram showing a flow of a liquid pressure when a breakinghydraulic pressure is supplied at the time of implementation of a TCSwith no operation of a break pedal in a case where the breakinghydraulic circuit according to the embodiment of the present inventionis used.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described indetail with reference to the accompanying drawings.

FIG. 1 is a diagram showing the configuration of a breaking hydrauliccircuit for controlling linear pressure reduction of a front/rear wheelbreaking liquid pressure and solving a difference in gradient of thefront/rear wheel breaking liquid pressure according to an embodiment ofthe present invention. The breaking hydraulic circuit according to theembodiment of the present invention includes a liquid pressure supplyline, supply path control valves, a liquid pressure return line, returnpath control valves, and a TCS liquid pressure supply line 14.

The liquid pressure supply line supplies a liquid pressure from a mastercylinder 1 to wheel cylinder 2 and 3 of wheels at the time of normalbreaking with no application of an ABS according to an operation of abreak pedal by a driver.

The supply path control valves are provided on the liquid pressuresupply line so as to control a flow of the liquid pressure to besupplied from master cylinder 1 to wheel cylinders 2 and 3 of thewheels.

The liquid pressure return line controls liquid pressures to bedischarged from wheel cylinders 2 and 3 at the time of pressurereduction control with the application of the ABS (Antilock BrakeSystem).

The return path control valves are normal close type solenoid valves andare provided on the liquid pressure return line so as to form atwo-system liquid pressure flow for separately passing the liquidpressures discharged from wheel cylinders 2 and 3 and then unifying theliquid pressures, such that a difference in pressure intensificationgradient according to a difference in generation time of a breakingpressure between front and rear wheels at the time of the pressurereduction control is eliminated.

TCS liquid pressure supply line 14 shares the liquid pressure supplyline so as to supply the liquid pressure of master cylinder 1 to wheelcylinders 2 and 3 through a high pressure switching valve 21, a motor 4,and a suction pump 5 at the time of implementation of a TCS for skidcontrol with no operation of the break pedal.

The breaking hydraulic circuit according to the embodiment of thepresent invention further includes a controller that controls the valvesprovided on the hydraulic circuit, motor 4, and suction pump 5 on thebasis of wheel speed information measured by a wheel speed sensor at thetime of normal breaking, the ABS pressure reduction control, or theimplementation of the TCS.

Although a case where the breaking hydraulic circuit according to theembodiment of the present invention is formed between master cylinder 1and wheel cylinders of front left and rear right wheels FL and RR isdescribed, a braking hydraulic circuit can be formed between mastercylinder 1 and wheel cylinders of front right and rear left wheels FRand RL. This braking hydraulic circuit also uses the same hydraulic lineand the same valves and suction pump. Motor 4 drives the suction pumpsrespectively mounted on both hydraulic circuits. That is, the samebreaking hydraulic circuits are formed between master cylinder 1 and thewheel cylinders of the front left and rear right wheels FL and RR andbetween master cylinder 1 and the wheel cylinders of the front right andrear left wheels FR and RL. Accordingly, hereinafter, one breakinghydraulic circuit will be described.

The liquid pressure supply line constituting the breaking hydrauliccircuit includes a head liquid pressure supply line 6 that extends froman outlet port of master cylinder 1, a front left wheel liquid pressuresupply line 7 that is connected to head liquid pressure supply line 6and wheel cylinder 2 of the front left wheel, and has a unidirectionalcheck valve CV so as to receive the liquid pressure from master cylinder1, and a rear right wheel liquid pressure supply line 8 that isconnected to head liquid pressure supply line 6 and wheel cylinder 3 ofthe rear right wheel, and has a unidirectional check valve CV so as toreceive the liquid pressure from master cylinder 1.

At this time, liquid pressure supply lines 6, 7, and 8 are connectedthrough branch pipes at joints and branches.

The supply path control valves provided on the liquid pressure supplyline include a front wheel supply path control valve 22 that is providedon front left wheel liquid pressure supply line 7 connected to headliquid pressure supply line 6 extending from the port of master cylinder1 so as to open/close the path, and a rear wheel supply path controlvalve 23 that is provided on rear right wheel liquid pressure supplyline 8 connected to head liquid pressure supply line 6 extending fromthe port of master cylinder 1 so as to open/close the path.

Here, control valves 22 and 23 all are normal open type solenoid valves.

The liquid pressure supply line is provided with a pressure settingvalve 20 (TCV) that, when the liquid pressure exceeds a prescribedliquid pressure, returns an excessive liquid pressure to master cylinder1 and keeps the entire hydraulic line to a constant liquid pressure.

The liquid pressure return line includes wheel cylinder liquid pressurereturn lines 9 and 10 that extend from wheel cylinder 2 of the frontleft wheel and wheel cylinder 3 of the rear right wheel and haveunidirectional check valves CV, respectively, a unified liquid pressurereturn line 11 that is formed by unifying said wheel cylinder liquidpressure return lines 9 and 10, a connection liquid pressure return line12 that is connected between unified liquid pressure return line 11 andsuction pump 5 and has a unidirectional check valve CV, and to which anaccumulator 27 for temporarily storing the liquid pressure is connected,and a discharge liquid pressure return line 13 that is connected to theliquid pressure supply line connected to master cylinder 1 so as toreturn the liquid pressure pumped from suction pump 5 toward mastercylinder 1.

Discharge liquid pressure return line 13 is connected to the liquidpressure supply lines that are respectively formed between mastercylinder 1 and wheel cylinders 2 and 3 of the front left wheel and rearright wheel. Joints are located between front wheel and rear wheelsupply path control valves 22 and 23 and pressure setting valve 20 (TCV)for keeping the breaking hydraulic circuit to a constant liquidpressure. This is to supply a braking liquid pressure to wheel cylinders2 and 3 through discharge liquid pressure return line 13 at the time ofTCS implementation.

The return path control valves provided on the liquid pressure returnline include front wheel and rear wheel return path control valves 24and 25 that are provided in wheel cylinder liquid pressure return lines9 and 10 respectively extending from wheel cylinders 2 and 3 of thefront left wheel and rear right wheel so as to open/close the paths, anda unified return path control valve 26 that is provided on connectionliquid pressure return line 12 connected to unified liquid pressurereturn line 11 extending from control valves 24 and 25 and being unifiedas a single line.

Here, front wheel and rear wheel return path control valves 24 and 25and unified return path control valve 26 all are normal close typesolenoid valves.

Liquid pressure supply lines 9, 10, 11, 12, and 13 are connected throughbranch pipes at joints or branches, if necessary.

TCS liquid pressure supply line 14 supplies the liquid pressure that isnot directly supplied from master cylinder 1 to wheel cylinders 2 and 3at the time of TCS implementation for skid control with no operation ofthe break pedal. TCS liquid pressure supply line 14 branches off fromthe liquid pressure supply line extending from the port of mastercylinder 1 and is connected to suction pump 5 to be connected to theliquid pressure return line.

At this time, TCS liquid pressure supply line 14 is connected toconnection liquid pressure return line 12 that connects unified returnpath control valve 26 and suction pump 5. The connection position is setat the back of unidirectional check valve CV provided on connectionliquid pressure return line 12. Then, the liquid pressure introducedthrough TCS liquid pressure supply line 14 does not reversely flow.

Accordingly, TCS liquid pressure supply line 14 forms a hydrauliccircuit for supplying the liquid pressure through the liquid pressurereturn line, suction pump 5, the liquid pressure supply line, and wheelcylinders 2 and 3.

TCS liquid pressure supply line 14 is provided with a high pressureswitching valve 21 that is opened so as to cause the liquid pressure ofmaster cylinder 1 to be supplied to wheel cylinders 2 and 3 throughmotor 4 and suction pump 5 at the time of the TCS implementation.

Hereinafter, the operation of the breaking hydraulic circuit accordingto the embodiment of the present invention will be described in detailwith reference to the accompanying drawings.

According to the embodiment of the present invention, for pressurereduction control at the time of the operation of the ABS, it ispossible to implement PWM SOL through a combination of NO SOL (NormalOpen type SOLenoid valve) and NC SOL (Normal Close type SOLenoid valve)as on/off type solenoid valves, not breaking hydraulic circuit controlusing PWM SOL (Pulse Width Modulation type SOLenoid valve).

That is, according to the embodiment of the invention, on the liquidpressure return line formed between the wheel cylinders and the mastercylinder, normal open type solenoid valves and normal close typesolenoid valves are provided so as to form a two-system liquid pressureflow for separately passing the liquid pressures to be discharged fromthe wheel cylinders and then unifying the liquid pressures. Further, anaccumulator that temporarily stores the discharged liquid pressure isprovided. Accordingly, a breaking operation can be rapidly performedduring the pressure reduction control of the ABS or TCS, and adifference in pressure intensification gradient according to adifference in generation time of a breaking pressure between the frontand rear wheels can be eliminated. Therefore, breaking unstability canbe solved.

The operation of the breaking hydraulic circuit according to theembodiment of the present invention will be described according tonormal breaking, the pressure reduction control according to theoperation of the ABS, and the TCS implementation for skid control.

First, at the time of normal breaking, as shown in FIG. 2, as the driveroperates the break pedal, a liquid pressure generated by a booster andmaster cylinder 1 is supplied to the wheel cylinders of the wheels FL,RR, FR, and RL through the breaking hydraulic circuit, therebyperforming breaking.

A detailed description will be given by way of a liquid pressure supplyline that is a hydraulic circuit from master cylinder 1 to wheelcylinders 2 and 3 of the front left wheel and rear right wheel, as shownin FIG. 2. That is, the liquid pressure generated by master cylinder 1is distributed, through head liquid pressure supply line 6, to frontleft wheel liquid pressure supply line 7 and rear right wheel liquidpressure supply line 8 connected to head liquid pressure supply line 6.Then, the liquid pressures distributed to front left wheel liquidpressure supply line 7 and rear right wheel liquid pressure supply line8 is introduced into wheel cylinder 2 of the front left wheel and wheelcylinder 3 of the rear right wheel, such that the breaking operation isperformed.

At this time, the controller that controls the hydraulic circuit opensfront wheel and rear wheel supply path control valves 22 and 23 of an NOSOL (Normal Open type SOLenoid valve) type respectively provided onfront left wheel and rear right wheel liquid pressure supply lines 7 and8, and closes front wheel and rear wheel return path control valves 24and 25 and unified return path control valve 26 of an NC SOL (NormalClose type SOLenoid valve) type provided in the liquid pressure returnline.

With no implementation of the TCS, motor 4 and suction pump 5 are notdriven, and high pressure switching valve 21 provided in TCS liquidpressure supply line 14 is kept in a closed state.

The controller performs pressure reduction control according to theoperation of the ABS. Specifically, the controller calculates adifference from a vehicle speed on the basis of information detected bya wheel speed sensor mounted on the wheel and then, if the differencedoes not fall within a prescribed control range, performs pressurereduction control in order to reduce the liquid pressures to be appliedto wheel cylinders 2 and 3 of the front left wheel and rear right wheel.At the time of the pressure reduction control, the controller drivesmotor 4 and suction pump 5 and performs on/off control of NO SOL typevalves 22 and 23 and NC SOL type valves 24, 25, and 26 provided in thehydraulic circuit. Accordingly, the pressure reduction control isperformed with no difference in pressure intensification gradientbetween the front and rear wheels.

Upon breaking, as shown in FIG. 2, the controller closes front wheel andrear wheel supply path control valves 22 and 23 of the NO SOL typeprovided in the liquid pressure supply line, and performs control toform a flow of the liquid pressure in the liquid pressure return linesuch that the liquid pressure is not supplied to wheel cylinders 2 and 3of the front left wheel and rear right wheel, to which the liquidpressure is supplied from master cylinder 1, any more, and apredetermined liquid pressure is discharged.

That is, as shown in FIG. 3, if the controller opens front wheel andrear wheel return path control valves 24 and 25 of the NC SOL typerespectively provided in the wheel cylinder liquid pressure return lines9 and 10 of wheel cylinders 2 and 3 of the front left wheel and rearright wheel, the liquid pressures to be discharged from wheel cylinders2 and 3 of the front left wheel and rear right wheel are collected inunified liquid pressure return line 11 through return path controlvalves 24 and 25.

Next, the liquid pressure introduced into unified liquid pressure returnline 11 passes through unified return path control valve 26 of the NCSOL type that is opened by the controller, and is stored, throughconnection liquid pressure return line 12, in accumulator 27 fortemporarily storing the liquid pressure. Then, in a state where thesupply of the liquid pressure is blocked, the internal liquid pressuresare discharged from wheel cylinders 2 and 3 of the front left wheel andrear right wheel to accumulator 27 through the liquid pressure returnline. Accordingly, a rotation speed of a wheel disk increases due to thereduced liquid pressures of wheel cylinders 2 and 3 of the front leftwheel and rear right wheel.

At this time, when the liquid pressure in the breaking hydraulic circuitexceeds a prescribed liquid pressure, the controller drives motor 4 andsuction pump 5 and returns the liquid pressure temporarily stored inaccumulator 27 and the liquid pressure discharged through the liquidpressure return line to master cylinder 1 through pressure setting valve20. Therefore, the liquid pressure in the breaking hydraulic circuit isconstantly kept to the prescribed liquid pressure.

Next, the controller calculates the difference from the vehicle speed onthe basis of information about the wheel speed detected by the wheelspeed sensor again. Then, if the rotation speed of the wheel excessivelyincreases, the controller performs the breaking operation again in orderto prevent the excessive increase of the rotation speed. Specifically,the controller drives motor 4 and suction pump 5 and supplies the liquidpressures to wheel cylinders 2 and 3 of the front left wheel and rearright wheel through the liquid pressure supply line.

That is, as shown in FIG. 4, the controller closes front wheel and rearwheel return path control valves 24 and 25 and unified return pathcontrol valve 26 of the NC SOL type provided in the liquid pressurereturn line. Simultaneously, the controller drives motor 4 and suctionpump 5 and opens front wheel and rear wheel supply path control valves22 and 23 of the NO SOL type provided in the liquid pressure supplyline. Then, the liquid pressure pumped from the liquid suction pump 5 isdistributed into front left wheel and rear right wheel liquid pressuresupply lines 7 and 8 and then the distributed liquid pressures arerespectively introduced to wheel cylinders 2 and 3 of the front leftwheel and rear right wheel.

Accordingly, wheel cylinders 2 and 3 of the front left wheel and rearright wheel perform the breaking operation again, and the wheel speed isreduced. Then, the controller acquires information about the wheel speedthrough the wheel speed sensor again, judges where or not to supply theliquid pressure for breaking or to discharge the liquid pressure forspeed reduction, and repeats the above-described operation, ifnecessary.

At the time of normal breaking or the implementation of the pressurereduction control according to the operation of the ABS, if theoperation of the break pedal by the driver is released, the controllerperforms control according to the break pedal release signal such thatthe liquid pressure in the breaking hydraulic circuit is kept constant.

That is, the controller that receives the break pedal release signalcloses front wheel and rear wheel supply path control valves 22 and 23of the NO SOL type provided in the liquid pressure supply line, andsimultaneously closes front wheel and rear wheel return path controlvalves 24 and 25 and unified return path control valve 26 of the NC SOLtype provided in the liquid pressure return line.

Accordingly, the liquid pressures of the wheel cylinders 2 and 3 of thefront left wheel and rear right wheel are discharged through wheelcylinder liquid pressure return lines 9 and 10 and unified liquidpressure return line 11. Then, the liquid pressures discharged to thedischarge liquid pressure return line return to master cylinder 1,excluding the liquid pressure for keeping the prescribed liquid pressureof the breaking hydraulic circuit.

At this time, the prescribed liquid pressure of the breaking hydrauliccircuit is kept through pressure setting valve 20. That is, only when aliquid pressure larger than the prescribed liquid pressure is formedthrough liquid pressure return lines 9, 10, 11, 12, and 13, pressuresetting valve 20 is opened so as to solve a difference from theprescribed liquid pressure, and then the liquid pressure larger than theprescribed liquid pressure returns to master cylinder 1. Accordingly,after breaking is released, the breaking hydraulic circuit is kept tothe prescribed liquid pressure.

Meanwhile, in a state where the driver does not operate the break pedal,when the TCS is implemented for skid or slip control, the controllercalculates a slip ratio of a tire using the vehicle speed informationinput through the wheel speed sensor, then drive motor 4 and suctionpump 5, and subsequently forms a hydraulic circuit between mastercylinder 1 and wheel cylinders 2 and 3 of the front left wheel and rearright wheel through TCS liquid pressure supply line 14 and front leftwheel and rear right wheel liquid pressure supply lines 7 and 8.

That is, as shown in FIG. 5, the controller closes front wheel and rearwheel return path control valves 24 and 25 and unified return pathcontrol valve 26 of the NC SOL type in the liquid pressure return line,opens front wheel and rear wheel supply path control valves 22 and 23 ofthe NO SOL type in the liquid pressure supply line, and opens highpressure switching valve 21 provided in TCS liquid pressure supply line14, thereby forming the hydraulic circuit. Through the hydraulic circuitformed in such a manner, the liquid pressure pumped by motor 4 andsuction pump 5 is introduced into wheel cylinders 2 and 3 of the frontleft wheel and rear right wheel, and thus the breaking operation isperformed.

At the time of the TCS implementation, when the wheel speed is reducedto a prescribed rotation speed or less on the basis of the slip ratioinformation of the wheel, the controller performs the pressure reductioncontrol for reducing the breaking liquid pressure. This is performed inthe same manner as at the time of the pressure reduction controlaccording to the operation of the ABS, and thus the description thereofwill be omitted.

Although the breaking operation and the pressure reduction control havebeen described using the hydraulic circuit formed master cylinder 1between wheel cylinders of front left wheel and rear right wheel FL andRR, a breaking hydraulic circuit that is to be formed between mastercylinder 1 and the wheel cylinders of front right wheel and rear leftwheel FR and RL is the same as the breaking hydraulic circuit that isformed between master cylinder 1 and wheel cylinders of front left wheeland rear right wheel FL and RR. Accordingly, the description of theoperation of the breaking hydraulic circuit that is formed betweenmaster cylinder 1 and wheel cylinders of front right wheel and rear leftwheel FR and RL will be omitted.

As described above, according to the embodiments of the presentinvention, on the liquid pressure return line that is formed between thewheel cylinders and the master cylinder, the normal open type solenoidvalves and the normal close type solenoid valves are provided so as toform the two-system liquid pressure flow for separately passing theliquid pressures to be discharged from the wheel cylinders and thenunifying the liquid pressures. Therefore, at the time of theimplementation of the ABS or the TCS, precise pressure reduction controlcan be preformed.

According to the embodiment of the present invention, the pressurereduction control according to the operation of the ABS or the TCS isimplemented using the normal close type solenoid valves. Therefore,costs for the development of a separate PWM type solenoid valve are notrequired, and thus an advantage can be obtained in terms of the costs.

According to the embodiment of the present invention, on the liquidpressure return line formed between the wheel cylinders and the mastercylinder, the normal close type solenoid valves are formed double so asto form a two-system liquid pressure flow, and the accumulator fortemporarily storing the discharge liquid pressure is provided.Therefore, the breaking operation can be rapidly performed during thepressure reduction control. As a result, the breaking operation can beperformed with no difference in pressure intensification gradientaccording to a difference in generation time of the breaking pressurebetween the front and rear wheels.

1. A breaking hydraulic circuit that controls linear reduction of afront/rear wheel breaking liquid pressure and solves a difference ingradient of the front/rear wheel breaking liquid pressure in a vehicle,the breaking hydraulic circuit comprising: a liquid pressure supply linethat supplies a liquid pressure from a master cylinder to wheelcylinders of wheels upon breaking according to an operation of a brakepedal by a driver and at normal breaking with no application of an ABS(Antilock Brake System); supply path control valves that are normal opentype solenoid valves and are provided on said liquid pressure supplyline to control a flow of a hydraulic pressure to be supplied from themaster cylinder to the wheel cylinders of the wheels; a liquid pressurereturn line that is formed to control liquid pressures to be dischargedfrom the wheel cylinders at a time of pressure reduction control with anapplication of the ABS; return path control valves that are normal closetype solenoid valves and are provided on said liquid pressure returnline to form a two-system liquid pressure flow that separately passesliquid pressures to be discharged from the wheel cylinders and thenunifies the liquid pressures, such that a difference in pressureintensification gradient is eliminated according to a difference ingeneration time of a breaking pressure between the front/rear wheels atthe time of the pressure reduction control; a TCS liquid pressure supplyline that shares the liquid pressure supply line and supplies the liquidpressure of the master cylinder to the wheel cylinders through a highpressure switching valve, a motor, and a suction pump at a time ofimplementation of a TCS (Traction Control System) to controltire-slipping with no operation of the break pedal; and a controllerthat controls the valves provided on the hydraulic circuit, the motor,and the suction pump based on wheel speed information measured by awheel speed sensor at the time of normal breaking, the ABS pressurereduction control, or the implementation of the TCS.
 2. The breakinghydraulic circuit as defined in claim 1, wherein said liquid pressuresupply line includes: a head liquid pressure supply line that extendsfrom the master cylinder; a front left wheel liquid pressure supply linethat is connected to said head liquid pressure supply line and a wheelcylinder of a front left wheel, and has a unidirectional check valve toreceive the liquid pressure from the master cylinder; and a rear rightwheel liquid pressure supply line that branches off from the head liquidpressure supply line, to which said front left wheel liquid pressuresupply line is connected, is connected to a wheel cylinder of a rearright wheel, and has a unidirectional check valve to receive the liquidpressure from the master cylinder.
 3. The breaking hydraulic circuit asdefined in claim 2, wherein a front wheel supply path control valve isprovided on said front left wheel liquid pressure supply line so as toopen/close the front left wheel liquid pressure supply line, and a rearwheel supply path control valve is provided on said rear right wheelliquid pressure supply line so as to open/close the rear right wheelliquid pressure supply line.
 4. The breaking hydraulic circuit asdefined in claim 2, wherein said liquid pressure supply line is providedwith a pressure setting valve that, when the liquid pressure exceeds aprescribed liquid pressure, returns an excessive liquid pressure to themaster cylinder to keep an entire hydraulic line at a constant liquidpressure.
 5. The breaking hydraulic circuit as defined in claim 4,wherein said pressure setting valve is provided on head liquid pressuresupply line before the front left wheel liquid pressure supply line andthe rear right wheel liquid pressure supply line branch off.
 6. Thebreaking hydraulic circuit as defined in claim 1, wherein said liquidpressure return line includes: wheel cylinder liquid pressure returnlines that extend from the wheel cylinder of the front left wheel andthe wheel cylinder of the rear right wheel, and have unidirectionalcheck valves, respectively; a unified liquid pressure return line thatis formed by unifying said wheel cylinder liquid pressure return lines;a connection liquid pressure return line that is connected between saidunified liquid pressure return line and a suction pump, and has aunidirectional check valve, and to which an accumulator that temporarilystores the liquid pressure is connected; and a discharge liquid pressurereturn line that is connected to the liquid pressure supply linesconnected to the master cylinder to return the liquid pressure pumpedfrom said suction pump to the master cylinder.
 7. The breaking hydrauliccircuit as defined in claim 6, wherein said discharge liquid pressurereturn line is connected to the liquid pressure supply lines that arerespectively formed from master cylinder to the wheel cylinders of thefront left wheel and rear right wheel.
 8. The breaking hydraulic circuitas defined in claim 7, wherein said discharge liquid pressure returnline is located between the front wheel and rear wheel supply pathcontrol valves provided on the liquid pressure supply lines and thepressure setting valve.
 9. The breaking hydraulic circuit as defined inclaim 6, wherein front wheel and rear wheel return path control valvesare provided in said wheel cylinder liquid pressure return lines,respectively, to open/close the wheel cylinder liquid pressure returnlines, and a unified return path control valve is provided on theconnection liquid pressure return line connected to said unified liquidpressure return line.
 10. The breaking hydraulic circuit as defined inclaim 1, wherein said TCS liquid pressure supply line branches off fromthe liquid pressure supply line extending from the master cylinder, isconnected to the liquid pressure return line connected to a previousstage of the suction pump, and is connected to the liquid pressuresupply line through the liquid pressure supply line extending from thesuction pump, thereby forming the hydraulic circuit.
 11. The breakinghydraulic circuit as defined in claim 1, wherein, as the breakinghydraulic circuit formed between said master cylinder and the wheelcylinders of the front left wheel and rear right wheel, a breakinghydraulic circuit is formed between the master cylinder and wheelcylinders of a front right wheel and a rear left wheel, and the suctionpumps respectively provided in said breaking hydraulic circuits aredriven by a single motor.